Generally, holographic recording records information in a recording medium by way of holography by superposing an information beam bearing image information and a reference beam in the recording medium, and writing a generated interference fringe pattern in the recording medium. For reproducing the stored information, the reference beam is applied to the recording medium to reproduce the image information based on the diffraction caused by the interference fringe pattern.
One process of recording an interference pattern in a recording medium is a so-called “two-beam interference process” for guiding an “information beam” and a “reference beam” to two optical paths having different irradiation angles and applying the beams separately to the recording medium to record an interference pattern in the recording medium (see, for example, Patent Document 1).
According to the two-beam interference process, a laser beam emitted from a laser beam source is divided into two laser beams (a first laser beam and a second laser beam) by a beam splitter, for example. The first laser beam is modulated into an information beam with an image by a spatial modulator, and the second laser beam is converted into a reference beam by adjusting its irradiation angle with an angle adjusting mirror. The information beam and the reference beam are applied across each other on the recording medium.
This process is problematic in that it requires an optical system for dividing the laser beam into the two laser beams, an optical system for converging the two laser beams onto the recording medium, and an optical system for adjusting the irradiation angle of one of the laser beams, resulting in a large-size recording and reproducing apparatus. In addition, the process has to meet various requirements, e.g., the recording medium requires very high smoothness and parallelism.
Other background art includes Patent Documents 2 and 3, for example.
There has also heretofore been proposed a recording process (colinear process) for recording information in an optical information recording medium by guiding an information beam and a reference beam to the same optical path to turn them into a recording laser beam, and applying the recording laser beam and a servocontrol laser beam to the optical information recording medium (see, for example, Patent Document 4 and Non-patent Document 1).
The proposed process needs much simpler optical systems than the above two-beam interference process, and hence makes an apparatus required to perform the process smaller in size and lower in cost. Furthermore, the servo technology utilized for CDs and DVDs may be employed to form an interference pattern at desired addresses depending on the eccentricity and surface fluctuations of a rotating optical disk.
For reproducing the information that has been recorded on the recording medium by the proposed process, a reproducing reference beam having the same pattern as the recording reference beam is applied to the recording medium. When the reproducing reference beam is applied to the recording medium, since an area to which the reference beam is applied has been modified by the information beam applied to record the information, the modified area reflects the reproducing reference beam as a reproducing beam representing the information beam. The reproducing beam is separated from the optical path for the reproducing reference beam, and detected by a solid-state imaging device which converts the reproducing beam into an electric signal.
According to the proposed process, the reproducing reference beam and the servocontrol laser beam are applied to the recording medium. The servocontrol laser beam needs to reach servocontrol information provided in the recording medium. As it is necessary to prevent the reproducing reference beam from reaching the servocontrol information, a selective reflecting laser is provided in the recording medium.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-018868    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-177958    Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-502918 (PCT)    Patent Document 4: Japanese Laid-Open Patent Publication No. 2005-032308    Non-patent Document 1: Nikkei Electronics issued on Jan. 17, 2005, p. 105-p. 114